Species Interactions, Food Webs, and
Ecological Communities

An ecological community is defined as
a group of actually or potentially interacting species living in the same
place. A community is bound together by the network of influences that
species have on one another. Inherent in this view is the notion that whatever
affects one species also affects many others -- the "balance of nature".
We build an understanding of communities by examining the two-way, and
then the multi-way, interactions involving pairs of species or many species.

type of interaction

sign

effects

mutualism

+/+

both species benefit from interaction

commensalism

+/0

one species benefits, one unaffected

competition

-/-

each species affected negatively

predation, parasitism, herbivory

+/-

one species benefits, one is disadvantaged

Food webs are graphical depictions
of the interconnections among species based on energy
flow . Energy enters this biological web of life at the bottom of the
diagram, through the
photosynthetic
fixation of carbon by green plants. Many food webs also gain
energy inputs through the decomposition of organic matter, such as decomposing
leaves on the forest floor, aided by microbes. River
food webs in forested headwater streams are good examples of this.

Energy moves from lower to higher
trophic (feeding) levels by consumption: herbivores consumes plants,
predators consume herbivores, and may in turn be eaten by top predators.
Some species feed at more than one tropic level, hence are termed omnivores.
Figure 1 provides a simplified model of such a food web.

Generalized food web. A
food web is an assemblage of organisms, including producers, consumers and
decomposers, through which energy and materials may move in a community

We can look at this food web in two
ways. It can be a diagram of the flow of energy (carbon) from plants to
herbivores to carnivores, and so on. We will take this approach when we
examine energy
flow in ecosystems. In addition, members of a
food
web may interact with one another via any of the four interaction types
named above. An interaction between two species in one part of the web can
affect species some distance away, depending on the strength and sign of the
inter-connections. Often, adding a species (as when an exotic species invades a
new area) or removing a species (as in a local extinction) has surprisingly
far-reaching effects on many other species. This is due to the complex
inter-connections of species in ecological webs.

Ecologists use the following terms
to describe various categories of the effects of a change (in abundance,
or presence vs absence) of one species on another.

Direct effects refer to the impact
of the presence (or change in abundance) of species A on species B in a
two-species interaction.

Indirect effects refer to the
impact of the presence (or change in abundance) of species A on species
C via an intermediary species (A --> B --> C).

Cascading effects are those which
extend across three or more trophic levels, and can be top-down (predator
--> herbivore --> plant) or bottom-up (plant --> herbivore --> predator).

Keystone species are those which
produce strong indirect effects.

The keystone species concept is
one of the best-known ideas in community ecology. Although it is
true that many species potentially interact with one another in a food
web such as depicted in Figure 1, in nature there are big players and little
players. The biggest players of all are referred to as keystone species.
This is a species whose presence or absence, or substantial increase or
decrease in abundance, profoundly affects other species in the community.
Evidence usually comes from experiments in which one species is added to
or removed from a community. The name derives from the center stone
in an arch supporting its weight by inward-leaning stones. Removal
of the keystone causes the arch to collapse.

In the rocky inter-tidal zone of
Washington state, and in other, similar areas, starfish have been shown
to be keystone species
The entire community lives on relatively vertical rock faces in the wave-swept
inter-tidal zone. The community of marine invertebrates and algae
are adapted to cling or adhere to the rock face, where most fed upon the
small animal life suspended in the water (plankton). A bivalve, the mussel
Mytilus, is superior at attaching to rock faces, making it the competitive
dominant. A starfish (Pisaster) is an effective predator of the mussels,
making space available for other species, and consequently is critical
to maintaining a diverse biological community.

Instances are known where a predator
so strongly suppresses its prey (herbivores), that the trophic level below
(plants) benefits because it is released from the pressures of herbivory.
Such “top-down” trophic cascades, where the community looks more or less
‘green’ depending on the abundance of predators, are well-known in lakes.
We also know of examples where fertilizing a system, which increases plant
growth, results in more predators, through the increase in abundance of
herbivores. This is a “bottom-up” trophic cascade.

Our understanding of these complex
species interactions gives substance to the popular phrase, the “balance
of nature”. One can also appreciate how a human-induced removal of
one species (an extinction event) or the addition on one species (invasion
of a community by a non-native species) could result in harm to many additional
species, a topic we will consider in the
second semester.

We will gain a fuller appreciation
of the complex, multi-way interactions among species as we proceed through
this series of lectures. However, we can fully appreciate the complexity
of these multi-way interactions, it is helpful to first understand the
nuances of the various two-way interactions. We will develop our understanding
of species interactions in ecological communities based on these building
blocks.

Mutualistic Interactions

A mutualism is an interaction where
both sides benefit. Pollination is a common mutualistic interaction. The
plant gains gamete transfer, the animal gets nectar (and also pollen).

Facultative mutualisms are beneficial
but not essential to survival and reproduction of either party. Obligate
mutualisms are those that are essential to the life of one or both associates.
We will examine an example of each.

A fascinating facultative mutualism
involves the Boran people of Africa, and a bird known as the honey guide.
According to rock paintings, humans have collected honey in Africa for
20,000 years. Human
hunting parties are often joined by the greater
honeyguide (Indicator indicator), which leads them to bee colonies.
In unfamiliar areas, the average search time was 8.9 hr when unguided,
but only 3.2 hrs when guided by the bird. Borans use fire and smoke to
drive off the bees, break open the nest and remove the honey, but leave
larvae and wax behind. The bird gains access to larvae and wax. The use
of fire and smoke reduces the bird's risk of being stung, and humans increase
accessibility of nests. According to the Borans, the honeyguide informs
them of: direction, from the compass bearing of bird flight; distance,
from the duration of the bird's disappearance and height of perch; and
arrival, by the "indicator call". Birds and Borans can survive without
the other, but each benefits from this facultative mutualism.

A mutualism between
certain ants and a small tree, the acacia, provides an excellent example
of an obligate mutualism. This particular system has been extensively studied
in Costa Rica. The acacia provides a number of benefits to the ants, including
shelter (hollow thorns), protein (beltian bodies at tip of leaflets), nectar
(secreted near base of leaves). The ant (Pseudomyrmex) provides
several forms of protection. It attacks and removes herbivorous insects,
It also removes vines that might overgrow the acacia, and kills the growing
shoots of nearby plants that might become competitors. It clears away leaf
litter from near the plant, and since the acacia grows in a seasonally
dry environment where it occasionally is threatened by fire, the ant's
activities protect the tree from
fire damage as well.

Pseudomyrmex Ants attack
a Katydid placed on an Acacia Plant

Many other examples of mutualisms
may be familiar to you.

Gut symbionts in herbivores: mammals
can't digest cellulose

endosymbiosis and the origin of eukaryotic
cells: mitochondria, flagella, chloroplasts are thought to be derived from
free-living bacteria

pollination systems

the coral polyp and its endosymbiont
"alga" (actually a dinoflagellate)

Commensalism

When one species benefits, and the other
species is neither benefited nor harmed, the interaction is "+/0". In the
southeastern US and in South America, it is common to see egrets in cattle
pastures. They follow the cattle, eating insects that are dislodged or
forced to fly as cattle graze in the field. One might suppose that egrets
benefit cattle, by consuming insects that might compete with cows for food.
The interaction would be a mutualism if this was demonstrated (but it seems
a bit far-fetched). Assuming no benefit to the cattle, this is a commensalism.
It often is the case, as this example illustrates, that we aren't sure
if the interaction is "+/O" or "+/+".

The clown fish and anemone also illustrates
this point. The clown fish hides from enemies within the stinging tentacles
of a sea anemone, to which the clown fish is immune. Some report this interaction
as a mutualism, arguing that the clownfish drops scraps of food into the
mouth of the anemone. Careful studies have failed to find much support
for any benefit to the anemone, so this appears to be a commensalism.

Summary

Species interactions within ecological
webs include four main types of two-way interactions: mutualism, commensalism,
competition, and predation (which includes herbivory and parasitism). Because
of the many linkages among species within a food web, changes to one species
can have far-reaching effects. We will next examine competition and predation,
and then return to a consideration of more complicated indirect and cascading
effects.